The present invention relates to a method and apparatus for reducing electrostatic discharge (ESD) during testing of integrated circuits and, more particularly, to a method and apparatus for virtually eliminating ESD damage during temperature cycling and thermal shock testing of integrated circuits.
Temperature cycling and thermal shock testing are two types of test performed on most integrated circuits (as well as circuit boards) during an initial "design qualification" (i.e., "prove-in") of the device or board. The testing may be accomplished by placing the item to be tested (i.e., the device or board) in a highly conductive, stainless steel perforated basket that is alternately subjected to extreme "hot" and "cold" baths, with high circulation to insure a uniform temperature within the bath. The temperatures may be, for example, 155.degree. C. and -65.degree. C., respectively, for the baths. In a typical test arrangement, the item being tested may be alternated up to 1000 cycles between the two temperatures. Submersion time at a particular temperature may be, for example, 15 minutes per cycle.
The bath itself may comprise chemicals such as liquid nitrogen, water, or a commercially available liquid such as Flourinet. One problem present in the prior art testing arrangement is the static potential created during the testing as a result of factors such as the chemical's resistance, the duration of the test, the rate of circulation and humidity within the test chamber. Static-sensitive integrated circuit devices may be exposed to as much as 10,000 volts during a typical thermal shock or temperature cycling operation.
Temperature cycling tests may also involve "air-to-air" (or any other gaseous environment) testing, using a pair of chambers maintained at two predetermined temperature extremes (such as 155.degree. C. and -65+ C.). Electrostatic potential will also build up in this environment, with the capability to severely damage the parts being tested. "Thermal shock" testing may be defined as quickly changing the ambient temperature of the part being tested. Again, thermal shock testing results in the build up of static charge within the structure supporting the device being tested.
Prior attempts to reduce ESD damage to items being tested have included using alternative chemicals, or modifying the testing apparatus to reduce the circulation of the bath. Neither of these proposed solutions has been successful in reducing the static potential (and hence the ESD damage) to an acceptable level.